An aerator is an attachment which when attached to faucets or fixtures controls the flow of the water without affecting or changing the water pressure. Aerators are used to help save water by reducing the amount of water flowing.
To do so, it behaves like sieve which separates a single flow of water into multiple streams allowing air to be mixed with the stream. This air-water mixture results in reduced water consumption (since there is less space for water to flow when mixed with air).
On an average, using an aerator helps save approximate 60% water consumption.